1. Field of the Invention
The present invention relates to a method for measuring performance parameters of an infrared focal plane array module and a method for detecting bad pixels of an infrared focal plane array module, and, more particularly, to a method with enhanced accuracy in measuring performance parameters of an infrared focal plane array module and a detection method capable of accurately identifying bad pixels.
2. Description of the Related Art
Infrared light is invisible to the human eyes while the energy of infrared light radiated from an object can be measured by an infrared detector. The voltage values and current values obtained from the measurement of the infrared detector can be converted into a gray level image or a virtual color image to be displayed on a display for the human eyes to see. Although visible light images can be seen by the human eyes only when there is sufficient background light source, instead of being subject to limitations demanded by background light source or surrounding weather condition, clear infrared images can be obtained by using an infrared detector to receive energy of infrared light radiated from an object. With reference to FIG. 12, a common and prevailing way of using an infrared imaging camera 81 is to connect the infrared imaging camera 81 to a computer 82. The infrared imaging camera 81 faces an object 83. The computer 82 has an analog image acquisition card (not shown) equipped with an 8-bit NTSC (National Television System Committee) interface for rapidly acquiring infrared images of the object 83 taken by the infrared imaging camera 81, which can be used for measurement of performance parameters of the infrared imaging camera 81 and detection of bad pixels in the infrared images.
An infrared detector, such as an infrared focal plane array, embedded in the infrared imaging camera 81 is formed by an arrangement of multiple pixels. When the infrared detector performs infrared detection of the object 83, such as a flat black body field, because infrared signals radiated from the object 83 are rather accurate and uniform, responsivity values from the output of each pixel in the infrared focal plane array should be identical. In reality, the responsivity values from the output of each pixel in the infrared focal plane array are not uniform for the sake of the semiconductor manufacturing process or the nature of semiconductors. With reference to FIGS. 13 and 14, an original image of an infrared focal plane array is shown and has two different response areas. The dark gray areas on a left side and an upper left corner of FIG. 13 and on an upper portion of FIG. 14 have lower responsivity values. The light gray areas on the lower right corner of FIG. 13 and on the lower left corner of FIG. 14 have higher responsivity values. Clustered bad pixels, single bad pixel and rows of bad pixels are also marked on FIGS. 13 and 14. The different response areas (with higher responsivity values and lower responsivity values) and bad pixels all increase spatial noise and temporal noise of the infrared focal plane array and affect imaging quality of the infrared focal plane array to result in errors in assessment and analysis of performance index parameters, such as minimum resolvable temperature difference, spatial resolution, noise equivalent temperature difference, operability and the like.
Currently, methods for defining bad pixels include offset value method, extreme value method and gain value method. The offset value method is based on an offset table generated by applying two-point correction to the infrared focal plane array and defines a pixel as a bad pixel when an offset value of the pixel is beyond a percentage range of a mean value of the offset table (e.g. ±30%). The offset value method is not appropriate for infrared focal plane array with two or more than two response areas. With reference to FIG. 15A, a histogram defining a relationship between offset value and number of pixel is shown. Any pixel with an offset value beyond ±30% of the mean value (0.7 and 1.3) is considered as a bad pixel. Locations of bad pixels defined by the offset value method are shown on FIG. 15B, wherein black points instead of white points are defined as the bad pixels. From FIGS. 15A and 15B, the offset value method is not good for infrared focal plane array with two or more than two response areas. The two-point correction serves to correct the gain value and the offset value of each pixel such that the responsivity value of each pixel can be corrected and arranged on a same line.
The extreme value method first takes images of the flat black body field using infrared focal plane array, captures several original infrared images, takes a mean value of gray level values of a pixel array, and identifies a pixel as a bad pixel when an offset value of the gray level value of the pixel is beyond a percentage range of the mean value of the gray level values of the pixel array (e.g. ±25% or ±30%). For example, when the mean value of the gray level values of the pixel array is 7172, if the percentage range ±25% is taken to identify bad pixels, the pixels with gray level value higher than 8965 and lower than 5379 are considered as bad pixels. However, when the infrared focal plane array has two or more than two different response areas, if the mean value of one of the response areas is taken as a criterion, pixels in the other response area could be determined as bad pixels to cause issue of image distortion for the sake of different ranges or mean value of the gray level values in the other response area.
With reference to FIGS. 16A and 16B, the gain value method is based on a gain table generated by applying two-point correction to the infrared focal plane array and defines a pixel as a bad pixel when a gain value of the pixel is beyond a percentage range of a mean value of the gain table (e.g. ±30%). With reference to FIG. 16A, a histogram defining a relationship between gain value and number of pixel is shown. Any pixel with a gain value beyond ±30% of the mean value (0.7 and 1.3) is considered as a bad pixel. Locations of bad pixels defined by the gain value method are shown on FIG. 16B. From FIGS. 16A and 16B, the gain value method is not good for infrared focal plane array with two or more than two response areas.
As the foregoing methods have drawbacks in terms of accuracy of measuring performance parameters and bad pixel detection, how to increase the accuracy in measuring and analyzing infrared images and in non-uniformity correction and accurately define locations and number of bad pixels becomes a subject to be improved.